In a disk recording and/or reproduction system an objective of the servo system which controls the radial position of the transducer on the disk, is to keep the transducer over the center of a preselected track. In a disk file system, this is done as the transducer reads position information from a track in the surface of the disk as the disks rotates. This position information is then used to develop a position error signal. The error signal is then fed back through a compensator into the drive motor for the transducer actuator to move the transducer in a direction to reduce the error.
The servo system includes a feed back servo loop. The position error signal is coupled into the feed back loop. Because of the finite response time of the feed back servo loop in correcting for disturbances, these disturbances or displacements cannot be totally eliminated. In the prior art, to aid the servo system in holding the transducer on track center in the presence of a physical disturbance or displacement, such as a shock force, an accelerometer has been found to be useful. An accelerometer mounted on the base plate of the disk file or compact disk player senses the resulting acceleration. Such acceleration will cause the disk to move relative to the transducer, resulting in an off track excursion of the transducer. The signal from the accelerometer is coupled to the actuator drive motor to cause a force to be applied to the transducer which, ideally, moves the transducer with the disk in the direction of the displacement, resulting in no net displacement of the transducer with respect to the track. This system has been in use for several years.
Typical prior art systems are found in U.S. Pat. Nos. 4,040,103; 4,138,121; 4,325,089; 4,532,802; 4,562,562; 4,573,087; 4,692,915; 4,862,298 and 4,947,093.
These patents are briefly reviewed below.
U.S. Pat. No. 4,040,103 describes a shock force compensating system in which, an accelerometer senses shock forces acting perpendicular to the gap between the head and the disk. Disturbance between the head and disk lags the shock by about one to ten milliseconds. Acceleration sensing is employed to provide lead time to minimize displacement.
U.S. Pat. No. 4,138,121 describes a control for a tone arm assembly in a record player. High fidelity reproduction is said to be obtained by employing a detector for detecting low and high frequencies in horizontal and vertical directions and using such detection to develop a signal which is fed back to cancel the vibrations.
U.S. Pat. No. 4,325,089 employs vibration sensing in an effort to safeguard magnetically recorded data. In this disclosure, unauthorized access to data is minimized by detectors which destroy the disk data by causing it to be passed through an electromagnetic field which erases the data. One detector of the plurality of detectors which are employed, is responsive to vibration caused by attempted physical access to the disk.
U.S. Pat. No. 4,532,802 employs accelerometers in a system for analyzing the interface between a recording disk and a read/write head. In one of its aspects, this system employs two accelerometers which ride piggy back on the transducer. One accelerometer is positioned at the leading edge of the transducer body and the other accelerometer is positioned at the trailing edge of the transducer body. These accelerometers are of the piezoelectric type. In their respective positions, these accelerometers sense vertical accelerations of the transducers and produce signals which when processed produce traces of the excursions including head crashes.
U.S. Pat. No. 4,562,562 describes a tracking system for controlling the radial position of a transducer on a disk in a disk file or drive. In the disclosed arrangement, a circuit is provided which detects the acceleration of the transducer in the rough searching or seek mode of the system. A signal developed by this acceleration detection circuit is applied to the actuator which positions the transducer or tracking element and powers the actuator in a direction so that the transducer error is substantially zero.
U.S. Pat. No. 4,573,087 describes a recording apparatus involving an electromagnetic device which senses a vibration of the recording head. When the recording head is stabilized a signal is produced which enables recording.
U.S. Pat. No. 4,692,915 describes a recording and reproduction apparatus employing a system in which sensitivity to acceleration is minimized. This is accomplished by providing three different accelerometers which respectively sense accelerations perpendicular to the recording tracks, accelerations perpendicular to the disk surface and accelerations parallel to the track direction. Excesses of acceleration sensed by an abnormality detector suspends transducing.
U.S. Pat. No. 4,862,298 describes a shock load detection device for a disk drive. Here there is described a cluster of piezoelectric sensors which sense accelerations in x, y and z directions, these being mutually perpendicular axes. The signals produced by these respective piezoelectric sensors are compared to threshold signals for the respective axes. When the acceleration signals exceed the threshold signals, reading or writing is interrupted.
U.S. Pat. No. 4,947,093 describes a shock resistant control system for a Winchester type of disk drive. Here, it is observed, that angular acceleration having a torque component acting about the axis of the disk stack causes circumferential shift of the tracks on the disk drive with respect to a transducer. This results in frequency modulation which is sensed. A phase comparator responsive to this modulation, controls the frequency of a voltage controlled oscillator in the system which shifts the phase to accommodate the frequency modulation which has taken place. In a rotary actuator drive of the type described, the transducer may also shift from track center as a result of the angular acceleration. This produces a position error signal which is coupled into the servo loop which controls the transducer.
The phase comparison signal is also, after filtering, coupled into the servo loop and functions to augment the positioning of the transducer by the servo. It is noted that acceleration sensing is not a feature of this particular disclosure.
One problem encountered in the implementation of prior art systems employing accelerometers of the type referenced above, is that it is expensive to obtain and maintain the necessary calibration of the accelerometer signal. Typically, low cost accelerometers like those economically feasible to this application, as manufactured, do not exhibit the same acceleration response and temperature response. Thus the signal gains at the source vary widely among the accelerometers. Augmenting this undesirable condition is the fact that other sources of signal gain in the signal path of the accelerometer to the head are subject to gain variation as well.
A second problem encountered in the manufacture and use of prior art systems of the type described, lies in testing the performance of the accelerometer in the servo system. It is expensive and impractical in high volume manufacture to perform an external shock or vibration test to evaluate the function (goodness) of the accelerometer shock corrective loop. Such a test requires a large mechanical shaker or other means for imparting external acceleration disturbances to the disk drive for the disk player for each test station.
There remains a need for a system in which accelerometers of different gain may be employed in disk recording and reproduction systems to function acceptably in the environment of use without the need for matched selection and extensive test and calibration of an assembled system.